This is one of those many reasons why I'm so excited for 1.13: Mojang has really worked hard to give us features within vanilla gameplay which are right now only available through mods. And I think that's a very good thing.

For example: on servers it's not uncommon to use the OpenInv mod which gives admins full access to a players inventory (including enderchests). Well... now we also got the /data command

Now, in case you're wondering what I did here...

I had to use 2 steps. First I used: /data get entity AyanamiKun SelectedItemSlot. This told me which of the 9 item slots in the hotbar Aya had selected. As you can see it was number 5, which basically means she had number 6 selected (Minecraft starts to count at 0).

So then I used another data command to tell me the 5th item in Aya's inventory: /data get entity AyanamiKun Inventory[5].

I had to use 2 steps. First I used: /data get entity AyanamiKun SelectedItemSlot. This told me which of the 9 item slots in the hotbar Aya had selected. As you can see it was number 5, which basically means she had number 6 selected (Minecraft starts to count at 0).

So then I used another data command to tell me the 5th item in Aya's inventory: /data get entity AyanamiKun Inventory[5].

You're not being consistent here. I suppose you meant the 6th item in her inventory?